BST2 is known to be a transmembrane glycoprotein that is expressed at high levels in various cancer cells including myeloma (Non-patent Documents 1 to 4). To date, BST2 antigen has been reported to include several splicing variants (Patent Documents 1 and 5). It is also known that of these variants, BST2D is expressed at high levels in tumors (Non-patent Document 1). It is also known that anti-BST2 monoclonal antibody exhibits cytotoxicity against cells by binding to BST2 expressed on the cell surface (corresponding to the molecular species BST2D) (Non-patent Document 2). Furthermore, there have been attempts to develop antitumor agents that target tumor cells in vivo based on this principle. For example, the agents have been demonstrated to produce a superior antitumor effect which is expected to shrink or eradicate tumors in model mice grafted with human myeloma cells (Non-patent Documents 3 and 4).    Patent Document 1: WO 1999/043803    Patent Document 2: WO 1998/035698    Patent Document 3: WO 2002/064159    Patent Document 4: WO 2005/034994    Patent Document 5: WO 2006/008886    Patent Document 6: WO 2006/013923    Non-patent Document 1: Goto T. et al., Blood 84(6): 1922-30, 1994    Non-patent Document 2: Ohtomo T. et al., Biochem Biophys Res Commun. 258(3): 583-91, 1999    Non-patent Document 3: Ozaki S. et al., Blood 90(8): 3179-86, 1997    Non-patent Document 4: Koishihara Y. et al., Blood 92(s): 107, 1998